In semiconductor fabrication processes, conventional sources of plasma source power, such as inductively coupled RF power applicators or capacitively coupled RF power applicators, introduce inherent plasma density non-uniformities into the processing. In particular, inductively coupled plasma sources are characterized by an “M”-shaped radial distribution of plasma ion density over the semiconductor workpiece or wafer. As device geometries have continued to shrink, such non-uniformities become more critical, requiring better compensation. Presently, the non-uniformity of an overhead inductively coupled source is reduced or eliminated at the wafer surface by optimizing the coil design and the ceiling-to-wafer distance of the chamber. This distance must be sufficient so that diffusion effects can overcome the effects of the non-uniform ion distribution in the ion generation region before they reach the wafer. Generally, for an inductively coupled plasma source power applicator (e.g., a coil wrapped around the side wall) located near the ceiling, a large ceiling-to-wafer distance is advantageous. However, a large ceiling-to-wafer distance can prevent the beneficial gas distribution effects of a ceiling gas distribution showerhead from reaching the wafer surface, due to diffusion over the large distance. For such large ceiling-to-wafer distances, it has been found that the gas distribution uniformity is not different whether a gas distribution showerhead is employed or a small number of discrete injection nozzles are employed. In summary, the wafer-ceiling gap is optimized for ion density uniformity, which may not necessarily lead to uniform gas distribution.
One limitation of such reactors is that not all process parameters can be independently controlled. For example, in an inductively coupled reactor, in order to increase reaction (etch) rate, the plasma source power must be increased to increase ion density. But, this increases the dissociation in the plasma, which can reduce etch selectivity and increase etch microloading problems, in some cases. Thus, the etch rate must be limited to those cases where etch selectivity or microloading is critical.
Another problem arises in the processing (e.g., etching) of multi-layer structures having different layers of different materials. Each of these layers is best processed (e.g., etched) under different plasma conditions. For example, some of the sub-layers may be best etched in an inductively coupled plasma with high ion density and high dissociation (for low mass highly reactive species in the plasma). Other layers may be best etched in a capacitively coupled plasma (low dissociation, high mass ions and radicals), while yet others may be best etched in plasma conditions which may be between the two extremes of purely inductively or capacitively coupled sources. However, to idealize the processing conditions for each sub-layer of the structure being etched would require different process reactors for each of the different sub-layers, and this is not practical.
Gas distribution is most effectively controlled by injecting the process gas into the reactor chamber through a gas distribution showerhead forming a portion of the ceiling overlying the wafer pedestal. Inductively coupled power distribution across the wafer is most effectively controlled by providing an inductively coupled power applicator (coil antenna) over the ceiling facing the wafer support pedestal. The problem is that if a ceiling gas distribution showerhead is combined with an overhead (ceiling) coil antenna, power from the coil antenna ionizes the process gas inside the showerhead, which degrades process control. Thus, there has seemed no way to combine the overhead gas distribution showerhead with an overhead coil antenna.